The present invention relates to improved threaded fasteners and threaded connections of a type having a greater capacity than conventional similar threaded connections to resist fatigue and tensile failures under the stresses encountered in use, and desirably also having a self-locking action. Certain features of the invention relate to improved methods for manufacturing such fasteners or for using them in forming a fatigue resistant threaded connection. The invention will be described primarily as applied to threaded nuts.
In a conventional threaded connection between an externally threaded screw and a nut disposed thereabout, failure of the threads of either the screw or nut under excessive tensile load conditions or as a result of fatigue normally occurs at the location of the inner or `bottom` turns of the nut thread, that is, the turns which are closest to the load bearing face of the nut. This is true because those bottom turns inherently must take a greater proportion of the total axial load exerted on the nut than do the other turns. Each successive turn of the nut thread above the `bottom` one takes less of the axial load than the next lower turn, with the result that most of the load is taken by the lower portion of the nut, and a relatively small part of the load is taken by the upper portion of the nut. This uneven distribution of the load forces among the different turns of the threads results in the discussed tendency for early fatigue and failure of the screw and nut at the location of the bottom turns of the nut thread.